How Much Light Do Indoor Plants Need From Lamps

Länge der Beleuchtung von Pflanzen

Control the development of your plant with your plant lamp by illuminating the plants for different lengths of time! Induce specifically the formation of flowers or optimize the vegetative growth – regardless of the season!

Dive briefly into chronobiology. Choose your plant from 5 categories. Learn how many hours of light your plant needs. Start with the right lighting duration and optimize your lighting times.

Enjoy reading and commenting!

How many hours of light do plants need?
Most plants need at least 10 hours of light to avoid going dormant. However, the optimal length of light depends on the cultivator’s goals. To achieve the main goals – growth or flowering – you must (usually) consider the critical day length of your plant.

Consider the “Critical Daylength
Critical daylength is the length of day or lighting duration within 24 hours that must be under or over for flower formation. There are species-specific differences.

For short-day plants, the critical day length is usually 12 hours.

For long-day plants, the critical day length is usually 14 – 16 hours.

Biologically, the duration of uninterrupted darkness is the determining factor, not the day length. Thus, long-day plants are short-night day plants and short-day plants are long-night day plants.

Short-day, long-day, and day-neutral plants.
Long-day plants flower when the light phase exceeds the critical daylength. In contrast, the growth phase is maintained when the light duration is below the critical daylength.

Short-day plants flower when the light phase falls below the critical daylength. The growth phase, on the other hand, is maintained when the illumination duration exceeds the critical daylength.

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Day-neutral plants are not photoperiodically controlled. Illumination time has no effect on flower formation, but depends on the developmental stage of the plant.

Qualitative and quantitative plants
Furthermore, a distinction is made between qualitative (obligatory) and quantitative (facultative) short & long day plants.

In qualitative short- and long-day plants, flower formation is subject to photoperiodic control. Extreme environmental conditions such as drought stress, cold and heat can still induce flower formation.

In quantitative short- and long-day plants, an appropriate photoperiod promotes flower formation. However, flower formation is also delayed when the photoperiod is unsuitable.

Find out in the next chapter if you have a short-day, long-day or day-neutral plant.

Wie lange eine Pflanzenlampe leuchten muss

Control the lighting time of your plant lamp

Now you know how long your plant lamp has to shine. Next, you need a timer to automatically adjust the appropriate lighting time to the required photoperiod.

Does long lighting lead to more yield?
As we already know, physiological development processes of plants can be controlled, among other things, by the length of the light phase. Flower formation can be induced or the vegetative phase can be maintained – apart from day-neutral plants.

To increase the yield of a plant, however, it is not necessarily decisive how long your plant light shines. Rather, yield depends on the amount of light energy (photons) your plant light emits. A weak plant light must shine longer than a strong one to generate the required amount of photons.

The parameter that determines this is the Daily Light Integral (DLI). The Daily Light Integral (DLI) describes the daily amount of photosynthetically active radiation in mol/m²/d that the plant receives or needs per day.

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Chronobiology and the temporal organization of the plant.
Chronobiology studies the temporal organization of physiological processes of living organisms.

The biological clock or the endogenous rhythms of plants within a day are investigated. The internal/endogenous clock serves as a timer for the plant and controls physiological processes. Light and darkness are zeitgebers and, as exogenous (external) stimuli, correct the internal clock to 24 hours. Entrainment – the synchronization of the internal clock – occurs.

What in humans is the wake-sleep rhythm, in plants is the circadian rhythm. The so-called circadian clock of the plant controls biological processes such as growth and reproduction.

Photoperiodism and adaptation to environmental changes
Photoperiodism is the dependence of physiological responses on the relative length of day and night. What is meant is the seasonal adaptation to developmental processes. The farther from the equator, the greater the seasonal variation in day length.

If different daylight lengths lead to changes in the developmental program of organisms, the physiological processes affected are subject to so-called photoperiodic control.

In most plants, development and physiological processes depend on the duration of the light phase or dark phase. Flower formation, the transition from the growth phase to the propagation phase, is particularly conspicuous. This process can be influenced and controlled in many plants by plant lamps.


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    Meet James Jones, a passionate gardening writer whose words bloom with the wisdom of an experienced horticulturist. With a deep-rooted love for all things green, James has dedicated his life to sharing the art and science of gardening with the world. James's words have found their way into countless publications, and his gardening insights have inspired a new generation of green thumbs. His commitment to sustainability and environmental stewardship shines through in every article he crafts.

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